A major difficulty which is encountered in the use of mercury-cathode electrolysis cells, such as those widely employed for the electrolysis of aqueous solutions of an alkali metal halide, for example sodium chloride, lies in the untimely and uncontrolled formation of a viscous accumulation adhering to the bottom of the cells and usually called "thick mercury" or "mercury butter".
The formation of these accumulations of thick mercury adhering to the bottom of the electrolysis cells interferes with the flow of the mercury film forming the cathode and consequently demands continuous surveillance of the position of the anodes and adjustment, at regular intervals, of their distance from the mercury film, to avoid the formation of chance short circuits. The gradual accumulation of thlck mercury on the bottom of the cells requires, moreover, that the bottom is cleaned at regular intervals and for this purpose it has been proposed to move a scraper over the bottom, at regular intervals, to detach from it the agglomerates of thick mercury (Belgian Patent No. 881,995 to Montedison SpA ; Central Patents Index, Abstracts Journal, London (Great Britain), Abstract 04648 W/03 : Japanese Patent Application 49,098,798 to Mitsubishi Chem.; ibid., Abstract 91,735 X/49 : Japanese Patent Application No. 76,041,040 to Mitsubishi Chem.). In these known processes, the movement of the scraper over the bottom of the cell is controlled so as to remove on each occasion all of the thick mercury which forms there.